Step 1
Unscrew the top of the vacuum breaker by hand; turn it to the left to remove. Only use pliers if necessary. The plastic can be broken easily.
Step 2
Pry up on one side of the cap by hand; it should pop off easily, as the lid off a shaving cream can. Try using both your thumbs to push up on the outer edge of the anti-siphon cap.
Step 3
Squeeze both sides of the cap simultaneously by hand and lift the top off. These types of caps are rare, but some do exist. They come off similarly to a pharmacy or safety medicine bottle top.